Methods for detecting and identifying a receiver in an inductive power transfer system

ABSTRACT

A method for detecting the presence of a receiver in an inductively coupled power transfer system having a transmitter and receiver. The method includes switching on a transmitter converter at a first frequency, measuring the inrush current and determining whether there is a receiver present. In another method, the inrush current is measured for a range of transmitter frequencies, and the variation in current is used to determine where there is a receiver present. In another method, the inrush current is measured when there is a change in voltage in the transmitter, and the variation in current is used to determine where there is a receiver present. In another method, the current supplied to the transmitter converter is measured over two transmitter frequencies, and the variation in current is used to determine where there is a receiver present. In another method, the current supplied to the transmitter converter is measured over two transmitter voltages, and the variation in current is used to determine where there is a receiver present.

This application is a Continuation of U.S. Ser. No. 15/493,857, filed 21Apr. 2017, which is a Continuation of U.S. Ser. No. 14/398,187, filed 31Oct. 2014, which is a National Stage Application of PCT/NZ2013/000068,filed 15 Apr. 2013, which claims benefit of Serial No. 599740, filed 2May 2012 in New Zealand and Ser. No. 61/696,341, filed 4 Sep. 2012 inthe United States and which applications are incorporated herein byreference. To the extent appropriate, a claim of priority is made toeach of the above disclosed applications.

FIELD OF THE INVENTION

The present invention is in the field of inductive power transfer (IPT)system. More particularly, the invention relates to methods fordetecting the presence of a receiver, and or identifying a receiver, tobe utilised in such systems.

BACKGROUND OF THE INVENTION

IPT systems are a well known area of established technology (forexample, wireless charging of electric toothbrushes) and developingtechnology (for example, wireless charging of handheld devices on a‘charging mat’). Typically, a primary side or transmitter generates atime-varying magnetic field with a transmitting coil or coils. Thismagnetic field induces an alternating current in a suitable receivingcoil that can then be used to charge a battery, or power a device orother load. In some instances, the transmitter coils or the receivercoils may be connected with capacitors to create a resonant circuit,which can increase power throughput and efficiency at the correspondingresonant frequency.

A common problem with IPT systems is controlling when the transmittershould be powered and when the transmitter should be switched off. Afurther problem arises when a non-receiver is brought into the range ofthe transmitter, and an unwanted current (and therefore heat) is inducedtherein. These non-receivers are typically known as parasitic loads.Lastly, it may be possible to detect the presence of a receiver, but itmay also be necessary to identify the receiver as being compatible withthe particular transmitter. Attempting to transfer power tonon-compatible receivers may result in inefficient power transfer (thus,undesired energy loss), or transmitter and/or receiver failure.

An obvious solution to the problems outlined above is to include amanually operated power switch with the transmitter. Though thisprovides a means for controlling when the transmitter should be powered,it undermines the convenience that is a goal of many IPT systems. Italso requires a user to manually switch off the transmitter when thereceiver is removed and does not accommodate any parasitic loads thatmay be introduced into the vicinity of the transmitter without theuser's knowledge.

Automatic systems for the detection and identification of receivers havebeen described in the prior art. For example:

-   -   Systems that rely on contact-based interaction between the        transmitter and receiver;    -   Systems that rely on communication signals sent between the        transmitter and receiver; and    -   Systems that use non-radioproximity sensors (eg light sensors)        to detect the physical presence of receivers.

All of these approaches rely on additional componentry to implement thedetection method. This adds complexity and cost to the design of IPTsystems. Perhaps more importantly, they tend to add bulk, whichfrustrates attempts to incorporate IPT systems into smaller devices suchas mobile phones, personal computers and the like.

To lessen these effects, it is known for IPT systems to utilise thepower transfer componentry for detection and identification as well (iemulti-purpose).

The drawbacks of these approaches are:

-   -   The power transfer may need to be reduced or completely        interrupted in order to carry out a detection method;    -   Where steady-state current is used an indicator of a receiver,        unloaded receivers may erroneously give a false result;    -   May be sensitive to component variations and noise; and    -   May be unable to identify whether a detected receiver is        compatible.

It is an object of the invention to provide methods for detecting oridentifying a receiver that do not require extensive additionalcomponentry to that required for inductive power, that produce accurateresults not sensitive to noise, that limit the time during which poweris not being transferred, that can positively identify a receiver or toat least provide the public with a useful choice.

SUMMARY OF THE INVENTION

The scope of the invention is as set out in the accompanying claims atthe end of this specification.

It is acknowledged that the terms “comprise”, “comprises” and“comprising” may, under varying jurisdictions, be attributed with eitheran exclusive or an inclusive meaning. For the purpose of thisspecification, and unless otherwise noted, these terms are intended tohave an inclusive meaning—i.e. they will be taken to mean an inclusionof the listed components which the use directly references, and possiblyalso of other non-specified components or elements.

Reference to any prior art in this specification does not constitute anadmission that such prior art forms part of the common generalknowledge.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings which are incorporated in and constitute partof the specification, illustrate embodiments of the invention and,together with the general description of the invention given above, andthe detailed description of embodiments given below, serve to explainthe principles of the invention.

FIG. 1 shows a block diagram of an IPT system; and

FIGS. 2a to 5 show sample data sets.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments of the present invention relate to methods for detecting oridentifying a receiver in an inductive power transfer (IPT) system. FIG.1 is a block diagram showing a general representation of an IPT system1. The IPT system includes a transmitter 2 and a receiver 3. Thetransmitter includes a converter 4 that is connected to an appropriatepower supply 5. In FIG. 1 this is shown as a converter that is connectedto a DC-DC converter 6 that is in turn connected to the mains powersupply. The converter may be a non-resonant half bridge converter or anyother converter adapted for the particular IPT system, such as apush-pull converter. The converter is configured to output analternating current of desired frequency and amplitude. The voltage ofthe output of the converter may also be regulated by the converter, theDC-DC converter or combination of both.

The converter 4 is connected to transmitting inductor(s) 7. Theconverter supplies the transmitting inductor(s) with an alternatingcurrent such that the transmitting inductor(s) generate a time-varyingmagnetic field with a suitable frequency and strength. In someconfigurations, the transmitting inductors can also be considered to bean integral part of the converter, but for the sake of clarity thisdescription will refer to them as distinct.

The transmitting inductor(s) 7 may be a suitable configuration of coils,depending on the characteristics of the magnetic field that are requiredin a particular application and the particular geometry of thetransmitter. In some IPT systems, the transmitting inductors may beconnected to capacitors (not shown) to create a resonant circuit.

FIG. 1 also shows a controller 8 within the transmitter 2. Thecontroller can be connected to each part of the transmitter. Thecontroller is adapted to receive inputs from each part of thetransmitter and produce outputs that control the way each part of thetransmitter operates. The controller may include a memory 9. Thecontroller is preferably a programmable logic controller that isprogrammed to perform different computational tasks depending on therequirements of the IPT system.

In addition to the features of a general IPT system 1 outlined above,FIG. 1 also shows a representation of a sensor 10. Such a sensor isadapted for sensing a particular operating characteristic of thetransmitter 2 and may be connected to other parts of the transmitteraccordingly. In FIG. 1, it is shown connected to the junction betweenthe DC-DC converter 6 and the converter 4, which is appropriate formeasuring the current being supplied to the converter. Of course, othersensors may be required and the invention is not limited in thisrespect.

In some embodiments of the invention described in more detail below, thesensor 10 is adapted to measure current. Those skilled in the art willappreciate that there are many possible types of sensors that areadapted for measuring current, and the invention is not limited in thisrespect. One example is a current sense resistor. It will be understoodthat an appropriate current sensor will be used that is able to measurethe desired current characteristic dependent upon the requiredfunctionality. This will be discussed in more detail later.

FIG. 1 also shows a receiver 3. The receiver includes receivinginductor(s) 11 that are suitably connected to receiver circuitry 12 thatin turn supplies power to a load 13. The load may be a battery. Thereceiver circuitry is adapted to convert the induced current into a formthat is appropriate for the load. In some IPT systems, the receivinginductors may be connected to capacitors (not shown) to create aresonant circuit.

There will now be described five embodiments of methods for detectingand or identifying receivers in an IPT system, or for detectingconducting non-receivers. Though these methods will be described inrelation to the IPT system 1 described in relation to FIG. 1, it will beunderstood that the methods may be adapted to work with any number ofappropriate IPT system configurations, and similarly IPT systems may beadapted to work with these methods, and the invention is not limited inthis respect.

Inrush Current Detection Method

According to one embodiment of the invention, the inrush currentdetection method begins with the transmitter in a standby mode. In thismode, the transmitter is controlled to draw minimum power. Periodicallythe transmitter switches from the standby mode to a detection mode todetect whether any receivers have come into the transmitting range ofthe transmitter. In a preferred embodiment of the invention, thetransmitter is configured to temporarily switch into the detection modeevery 2 seconds. Alternatively, the transmitter may already be in adetection mode if the inrush current detection method was preceded byanother detection method.

Upon switching into a detection mode, the controller controls theconverter so that it supplies the inductor with an initial highfrequency alternating voltage. Where the IPT system has resonantnetworks, the frequency should be non-resonant. Such a currenteliminates any residual DC biases that may be present in capacitors inthe IPT system. Eliminating such biases improves the reliability of thesubsequent steps of the method. Other methods may be used foreliminating DC biases in the system such as a voltage divider or areplacing the half bridge inverter with a full bridge inverter. In apreferred embodiment, the high frequency current is supplied for asufficient time interval until a steady state has been reached. In oneembodiment of the invention, this time period is of the order of ˜10 ms.

In the next step, the controller controls the converter so that itsupplies the inductor with an alternating current at a test frequency.In a preferred embodiment, this is a frequency that a receiver will havethe strongest inrush. This frequency may be at or about the frequency atwhich the transmitter is configured to transmit power. Those skilled inthe art will appreciate that this frequency is dependent on the circuitcomponents used in the transmitter. For typical IPT systems, this can befrom ˜100 kHz-˜1 MHz. In a preferred embodiment, the test frequency is,or near to, ˜150 kHz.

Upon supplying the current at the test frequency, there will be aninrush period during which transient currents will flow through circuitcomponents in the transmitter and any receiver that may be present. Theexistence of transient currents is a well-known phenomenon in circuits.However normally transient currents are ignored until the system reachesa steady state. Conversely, these transient currents form the basis ofthe inrush current detection method.

A sensor is configured for measuring the current supplied to theconverter. As shown in FIG. 1, the sensor 10 may be connected at thejunction between the converter 4 and the DC-DC converter 6. During theinrush period, the sensor measures the amplitude of the current beingsupplied to the converter. In one embodiment, the sensor measures thepeak amplitude during the inrush period. In one embodiment, the sensormay include a peak amplitude detection circuit for this purpose. Thecontroller may be configured to provide the inrush period as an inputinto the sensor. In an alternative embodiment, the sensor may measurethe current amplitude continuously during the inrush period, with themeasurements being provided as inputs into the controller, which isconfigured to determine the peak amplitude from this data.

The peak amplitude of the current is then provided to the controller.The controller is configured to determine whether the peak amplitudeexceeds a threshold. If there is no receiver present, then the peakcurrent will be that due to the transient current in the circuitcomponents in the transmitter only. However, if there is a receiverpresent, then the peak current will typically be higher due to thetransient currents in the receiver. Therefore, it is preferable that thethreshold is selected so that it is higher than any peak currents thatmay be due to transmitter components only. Preferably, this is achievedby calibrating the controller having regard to the particularconfiguration of the transmitter. It is possible that non-receivers mayalso affect the magnitude of the peak current measured during the inrushperiod. Therefore, it may also be necessary to select the threshold sothat it is high enough to exclude non-receivers, whilst still being lowenough to ensure that transient currents in receivers cause the measuredpeak amplitude to exceed the threshold.

In one embodiment of the invention, the sensor may measure multiple peakcurrents during the inrush period. The sensor and or controller may beconfigured to disregard peaks that are characteristic of thetransmitter. Again, this may be done through calibration of the sensorand or controller in light of the configuration of the transmitter.

If the measured peak current exceeds the threshold, then there is alikelihood that a receiver may be in the range of the transmitter, andthus the controller may then:

-   -   control the transmitter so that power is transferred to the        receiver;    -   control the transmitter according to further detection methods        to further verify the presence of a receiver; or    -   control the transmitter according to further identification        methods to identify the compatibility of a receiver with the        transmitter.

If the measured peak current falls below the threshold, then there is alikelihood that a receiver may not be in the range of the transmitter,and thus the controller may then:

-   -   control the transmitter according to further detection methods        to detect the presence of a receiver;    -   return the transmitter to the previously described standby mode;        or    -   switch the transmitter off        Frequency Sweep Detection Method

According to one embodiment of the invention, the frequency sweepdetection method begins with the transmitter in a standby mode asdescribed above under the inrush current detection method.Alternatively, the transmitter may already be in a detection mode if thefrequency sweep detection method was preceded by another detectionmethod.

Upon switching into a detection mode, the transmitter is controlledaccording to the inrush current detection method described above, upuntil the step when the peak amplitude of the current has been providedto the controller. Then, instead of determining whether the peakamplitude exceeds a threshold, the controller stores the peak amplitudein memory as well as the value of the test frequency.

Then, the transmitter is again controlled according to the inrushcurrent detection method described above, up until the peak amplitude ofthe current has been provided to controller, however this time at asecond test frequency. The controller stores the peak amplitude inmemory as well as the value of the second test frequency.

The above step is repeated for a plurality of test frequencies over arange of frequencies. This results in the memory having a record of peakamplitude currents measured during the inrush period for a range offrequencies. In a preferred embodiment, the range of frequencies isselected so as to be generally centred about the frequency that areceiver will have the strongest inrush. This frequency may be at orabout which the frequency at which the transmitter is configured totransmit power.

The controller then analyses the record to determine whether there is amaximum in the relationship between the peak amplitudes of the currentsand test frequencies. The controller determines whether there is amaximum by any suitable method of function analysis.

FIGS. 2a and 2b show two example sets of data. In the first data set inFIG. 2a , it can be seen that there is slight variation, but there is nodiscernible maximum. Conversely, the second data set in FIG. 2b shows adiscernible maximum 14. The controller may be configured to determinethat there is a maximum only if it is sufficiently large in the contextof the data set as a whole, such as that illustrated by the second dataset in FIG. 2b (ie not necessarily a maximum in the strict mathematicalsense).

The controller then determines parameters associated with the maximum.This can include determining the width of the maximum (such as thefull-width half-maximum metric), the height of the maximum and thefrequency at which the maximum occurs. Again, those skilled in the artwill appreciate that any suitable method of function analysis can beadapted and used by the controller for this purpose. The controller maysmooth or average results to improve the reliability of the analysis.Referring to FIG. 3, there is shown another data set that exhibits amaximum 15. In this instance, the maximum has a width of 32 kHz, aheight of 2.0, and occurs at 120 kHz.

In one embodiment, the analysis of the maximum may include thefollowing:

-   -   Subtract the baseline data from the measured data, so that any        result is as compared to a baseline;    -   Smooth and filter noise by conducting moving average of width        three (or similar) on the data points;    -   Locate the edges of the maximum by finding where the slope        changes polarity;    -   Locate the top of the maximum by finding the largest value        between the edges of the maximum;    -   Measure the height at the edges of the maximum;    -   Maximum width may be defined as the width between the maximum's        edges; and    -   Maximum height may be defined as the vertical distance from the        height at the higher of the maximum edges to the height at the        top of the peak.

If a receiver is present, then a maximum will result that exhibits someor all of these characteristics. For example, a resonant receiver willcause a maximum to occur at the resonant frequency of the receiver,since more transient current will flow when the transmitter is coupledwith a receiver that is resonating. Further the frequency at which themaximum occurs may identify the type of receiver. For example, thereceiver that causes the maximum in FIG. 3 to be produced is configuredto be resonant at 120 kHz. Since the resonant frequency of a receivercan be unique to a type of receiver, the controller may be able toidentify whether the receiver is compatible with the transmitter. Inother words, the maximum can be a resonance ‘signature’. Thus, it willbe understood that the frequency sweep detection method may be a methodfor detecting a receiver, but may also be for identifying a receiver.

The controller includes predetermined parameters against which itcompares the parameters of the maximum. For example, the predeterminedparameters may provide that the maximum must have or exceed a certainwidth and or a certain height, and or must occur at a certain frequency,frequencies or within a frequency range.

If the controller determines that the parameters of the maximum satisfythe predetermined parameters, then there is a likelihood that a receiver(detected) or compatible receiver (identified) may be in the range ofthe transmitter, or that a previously detected receiver may becompatible with the transmitter. Thus the controller may then:

-   -   control the transmitter so that power is transferred to the        receiver;    -   control the transmitter according to further detection methods        to further verify the presence of a receiver; or    -   control the transmitter according to further identification        methods to further identify the compatibility of a receiver with        the transmitter.

If the controller determines that the parameters of maximum do notsatisfy the predetermined parameters or determines that there is nomaximum, then there is a likelihood that a receiver may not be in therange of the transmitter or that a receiver may not be compatible withthe transmitter. Thus, the controller may then:

-   -   control the transmitter according to further detection methods        to detect the presence of a receiver;    -   return the transmitter to the previously described standby mode;        or    -   switch the transmitter off        Inrush Current Removal Detection Method

According to one embodiment of the invention, the inrush current removaldetection method begins with the transmitter in a power mode. In oneembodiment, the transmitter has already detected the presence of areceiver and has commenced transferring power to the receiver. In thepower mode, the transmitter is controlled to transfer power to areceiver. The converter and DC-DC converter will be controlled to supplyan alternating current to the inductor at an operating frequency and anoperating voltage.

Periodically the transmitter switches from the power mode to apower-detection mode (ie a mode for detecting receivers or parasiticloads whilst transferring power) to detect whether the receiver has beenremoved from the range of the transmitter. In a preferred embodiment ofthe invention, the transmitter is configured to temporarily switch intothe power-detection mode every 2 seconds. Upon switching into apower-detection mode, the controller controls the converter or DC-DCconverter so that the inductor is supplied with a slightly lowervoltage. The lower voltage should not be so small as to affect the ratedpower transfer from the transmitter to the receiver. In one embodiment,the smaller voltage is less than 4% smaller than the operating voltage.The system is then allowed to reach a steady state under the smallervoltage.

Next, the controller controls the converter or DC-DC converter so thatthe inductor is supplied with a higher voltage. The higher voltageshould not be so high as to affect the rated power transfer from thetransmitter to the receiver. In one embodiment, the higher voltage isless than 4% higher than the operating voltage. The higher voltage maybe the operating voltage.

Increasing the voltage in this way will result in an inrush period, andtransient currents will flow in circuit components present in thesystem. The transmitter measures the peak amplitude of the current beingsupplied to the converter as per the inrush current detection method,described above. The controller also determines whether the peakamplitude of the current exceeds a threshold according to the inrushcurrent detection method, described above Those skilled in the art willappreciate that the thresholds, time periods and characteristic peaksdescribed in relation to the inrush current detection method may need tobe modified to account for the fact the inrush current removal detectionmethod is undertaken during power transfer.

If the measured peak current exceeds the threshold, then there is alikelihood that a receiver may still be in the range of the transmitter,and thus the controller may then:

-   -   control the transmitter so that power is transferred to the        receiver, which may include returning the voltage of the        transmitter to the operating voltage.

If the measured peak current falls below the threshold, then there is alikelihood that a receiver may no longer be in the range of thetransmitter, and thus the controller may then:

-   -   control the transmitter according to further detection methods        to confirm the absence of the receiver;    -   return the transmitter to the previously described standby mode;        or    -   switch the transmitter off.

In another embodiment of the inrush current removal detection method,rather than initially decrease the voltage to a smaller voltage, thevoltage can be increased to a higher voltage, and the inrush currentmeasured at this stage. In this embodiment, the higher voltage shouldnot be so high as to affect the power transfer from the transmitter tothe receiver. In one embodiment, the higher voltage is less than 4%higher than the operating voltage.

In another embodiment of the inrush current removal detection method,rather than initially decrease the voltage to a smaller voltage, thevoltage is decreased to zero for a first test period and then returnedto the operating voltage. Upon increasing the voltage back to theoperating voltage the inrush current measured is measured according tothe above description.

The first test period is short enough such that switching the voltage tozero does not affect power transfer from the transmitter to thereceiver, particularly where the receiver is under load. In oneembodiment of the invention the first test period is ˜10 us. Such ashort test period may not allow the DC voltages in the receiver to decaysufficiently to measure a resultant transient current. So the test isrepeated over a series of increasing test periods up until a second testperiod, when there are resultant transient currents that are able to bemeasured. Alternatively, if no transient currents are detected at thesecond test period then it is determined that no receiver is present.Gradually increasing the test period in this way, allows the inrushperiod to be observed for the shortest ‘off-time’ (ie zero voltage)necessary, and thus where the receiver is under load, power transferwill not be interrupted.

Frequency Vary Detection Method.

According to one embodiment of the invention, the frequency varydetection method begins with the transmitter in a power mode. In oneembodiment, the transmitter has already detected the presence of aresonant receiver and has commenced transferring power to the resonantreceiver. In the power mode, the transmitter is controlled to transferpower to a resonant receiver. The converter and DC-DC converter will becontrolled to supply an alternating current to the inductor at anoperating voltage and an operating frequency, wherein the operatingfrequency is controlled to match the resonance of the receiver.

Periodically the transmitter switches from the power mode to apower-detection mode to detect whether a conducting non-receiver (ie aparasitic load has been introduced into the range of the transmitter. Ina preferred embodiment of the invention, the transmitter is configuredto temporarily switch into the power-detection mode every 2 seconds.

Upon switching into a power-detection mode, the sensor measures theaverage steady state current being supplied to the converter. Thecontroller stores this current value in memory as well as the value ofthe frequency.

The controller then adjusts the frequency to a test frequency. The testfrequency should be sufficient close to the operating frequency as tonot allow the receiver to fall out of resonance, and thus not affect therated power transfer from the transmitter to the receiver. In oneembodiment, the test frequency is less than 4% different from theoperating frequency.

The system is then allowed to reach a steady state under the newfrequency. The sensor measures the average steady state current beingsupplied to the converter. The controller stores this current value inmemory as well as the value of the test frequency.

The above step is repeated for a plurality of test frequencies over arange of frequencies. Those skilled in the art will appreciate that thisthen results in the memory having a record of currents measured over arange of frequencies. In a preferred embodiment, the range offrequencies is selected so as to be generally centred about theoperating frequency of the transmitter.

The controller then analyses the record to determine the relationshipbetween the steady state current and test frequencies. The controllerdetermines the relationship by any suitable method of function analysis.The controller may be configured to smooth or average the data toimprove the quality of the analysis. In a preferred embodiment, thecontroller performs linear regression analysis of the data to determinethe proportionality constant. FIG. 4 shows two example sets of data withcurrent on the vertical axis and frequency on the horizontal axis. Inthe first data set, it can be seen that there is slight variation, butgenerally the slope of the data is close to zero, ie the proportionalityconstant is, or is near, zero. Conversely, the second data set shows amuch more negative slope ie the proportionality constant is morenegative.

If a receiver is present, then the receiver will behave as a constantpower load, and thus slight variations in driving frequency will notaffect the amount of power being drawn. Therefore, the current beingdrawn by the converter will not vary significantly. Conversely, aconducting non-receiver (such as a piece of metal) will behave as aconstant resistance load, and thus for a transmitter coil being drivingat a constant voltage, slight increases in driving frequency willdecrease the amount of power being drawn. Therefore, the current flowinginto the converter will also decrease.

It will be appreciated that the proportionality constant described abovecan thus be used as an indicator as to the presence of a conductingnon-receiver. The controller may be calibrated so that only aproportionality constant with a value less than a certain threshold (iebeing ‘sufficiently negative’) is regarded as being indicative as to thepresence of a conducting non-receiver.

If the controller determines that the proportionality constant is notsufficiently negative, then there is a likelihood that a conductingnon-receiver may not be in the range of the transmitter, and thus thecontroller may then:

-   -   control the transmitter so that power is transferred to the        receiver.

If the controller determines that the proportionality constant issufficiently negative, then there is a likelihood that a conductingnon-receiver (ie a parasitic load) may be in the range of thetransmitter, and thus the controller may then:

-   -   control the transmitter according to further detection methods        to confirm the presence of a conducting non-receiver;    -   return the transmitter to the previously described standby mode;        or    -   switch the transmitter off        Voltage Vary Detection Method

According to one embodiment of the invention, the voltage vary detectionmethod begins with the transmitter in a power mode. In one embodiment,the transmitter has already detected the presence of a receiver and hascommenced transferring power to the receiver. In the power mode, thetransmitter is controlled to transfer power to a receiver. The DC-ACconverter and DC-DC converter will be controlled to supply analternating current to the inductor at an operating voltage and anoperating frequency.

Periodically the transmitter switches from the power mode to apower-detection mode to detect whether a conducting non-receiver (ie aparasitic load has been introduced into the range of the transmitter. Ina preferred embodiment of the invention, the transmitter is configuredto temporarily switch into the power-detection mode every 2 seconds.

Upon switching into a power-detection mode, the sensor measures theaverage steady state current being supplied to the converter. Thecontroller stores this current value in memory as well as the value ofthe operating voltage.

The controller then adjusts the voltage to a test voltage. In oneembodiment, this is achieved through control of the DC-DC converter. Inanother embodiment, the converter may be driven at a different dutycycle to change the drive voltage. The test voltage should be sufficientclose to the operating voltage as to not affect the rated power transferfrom the transmitter to the receiver. In one embodiment, the testvoltage is less than 4% different from the operating voltage.

The system is then allowed to reach a steady state under the newvoltage. The sensor measures the average steady state current beingsupplied to the converter. The controller stores this current value inmemory as well as the value of the test voltage.

The above step is repeated for a plurality of test voltages over a rangeof voltages. Those skilled in the art will appreciate that this thenresults in the memory having a record of currents measured over a rangeof voltages. In a preferred embodiment, the range of voltages isselected so as to be generally centred about the operating voltage ofthe transmitter.

The controller then analyses the record to determine the relationshipbetween the steady state current and test voltage. The controllerdetermines the relationship by any suitable method of function analysis.The controller may be configured to smooth or average the data toimprove the quality of the analysis. In a preferred embodiment, thecontroller performs linear regression analysis of the data to determinethe proportionality constant. FIG. 5 shows two example sets of data withcurrent on the vertical axis and voltage on the horizontal axis. In thefirst data set, it can be seen that there is a positive slope, ie theproportionality constant is positive. Conversely, the second data setshows a negative slope ie the proportionality constant is negative.

If a receiver is present, then the receiver will behave as a constantpower load, and thus slight increases in driving voltage will not affectthe amount of power being drawn. Therefore, the current being drawn bythe converter will decrease as applied voltage increases. (cf P=VI).Conversely, a conducting non-receiver (such as a piece of metal) willbehave as a constant resistance load, and thus slight increases indriving voltage will increases the amount of power being drawn.Therefore, the current flowing into the converter will also increase (cfI=V/R).

It will be appreciated that the proportionality constant described abovecan thus be used as an indicator as to the presence of a conductingnon-receiver. The controller may be calibrated so that only aproportionality constant with a value above a certain threshold (iebeing ‘sufficiently positively’) is regarded as being indicative as tothe presence of a conducting non-receiver.

If the controller determines that the proportionality constant is notsufficiently positive, then there is a likelihood that a conductingnon-receiver may not be in the range of the transmitter, and thus thecontroller may then:

-   -   control the transmitter so that power is transferred to the        receiver.

If the controller determines that the proportionality constant issufficiently positive, then there is a likelihood that a conductingnon-receiver (ie a parasitic load) may be in the range of thetransmitter, and thus the controller may then:

-   -   control the transmitter according to further detection methods        to confirm the presence of a conducting non-receiver;    -   return the transmitter to the previously described standby mode;        or    -   switch the transmitter off.

While the present invention has been illustrated by the description ofthe embodiments thereof, and while the embodiments have been describedin detail, it is not the intention of the Applicant to restrict or inany way limit the scope of the appended claims to such detail.Additional advantages and modifications will readily appear to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details, representative apparatus andmethod, and illustrative examples shown and described. Accordingly,departures may be made from such details without departure from thespirit or scope of the Applicant's general inventive concept.

The invention claimed is:
 1. A method for detecting a receiver in aninductively coupled power transfer system having: a coil for generatingan alternating magnetic field; a converter for supplying an alternatingcurrent to the coil; and a sensor for measuring an indication of thecurrent supplied by the converter; the method comprising: operating theconverter in a power delivery mode to deliver power to an inductivepower receiver via the coil; switching the converter to a powerdetection mode for detecting inductive power receivers or parasiticloads while transferring power, wherein the power detection modecomprises: operating the converter so as to vary at least one of avoltage or frequency of alternating current supplied to the coil to aplurality of test voltages or test frequencies; using the sensor,detecting a change in current supplied to the coil associated with thevarying of at least one of the voltage or frequency of alternatingcurrent supplied to the coil to the plurality of test voltages or testfrequencies; performing a regression analysis of the change in currentassociated with the plurality of test voltages or test frequencies todetermine a proportionality constant relating the change in current tothe variation of at least one of a voltage or frequency of alternatingcurrent supplied to the coil; and detecting a receiver or a parasiticload based on the proportionality constant satisfying one or morepredetermined parameters.
 2. The method of claim 1 wherein operating theconverter so as to vary at least one of a voltage or frequency suppliedto the coil comprises adjusting the frequency to one or more testfrequencies.
 3. The method of claim 2 wherein each of the one or moretest frequencies is less than 4% different than an operating frequencyin the power delivery mode.
 4. The method of claim 2 wherein: detectinga change in current supplied to the coil comprises: measuring averagesteady state current supplied at the power delivery frequency; storingthe measured average steady state current and associated power deliveryfrequency; measuring average steady state current supplied at the one ormore test frequencies; and storing the measured average steady statecurrent and associated test frequencies; and wherein performing aregression analysis of the change in current comprises: determining arelationship between steady state current and frequency.
 5. The methodof claim 2, further comprising: detecting a receiver based on theproportionality constant being greater than a threshold.
 6. The methodof claim 2, further comprising: detecting a parasitic load based on theproportionality constant being less than a threshold.
 7. The method ofclaim 1 wherein operating the converter so as to vary at least one of avoltage or frequency supplied to the coil comprises adjusting thevoltage to one or more test voltages.
 8. The method of claim 7 whereineach of the one or more test voltages is less than 4% different than anoperating voltage in the power delivery mode.
 9. The method of claim 7wherein: detecting a change in current supplied to the coil comprises:measuring average steady state current supplied at the power deliveryvoltage; storing the measured average steady state current andassociated power delivery voltage; measuring average steady statecurrent supplied at the one or more test voltages; and storing themeasured average steady state current and associated test voltages; andwherein performing a regression analysis of the change in currentcomprises: determining a relationship between steady state current andvoltage.
 10. The method of claim 7, further comprising: detecting areceiver based on the proportionality constant being less than athreshold.
 11. The method of claim 7, further comprising: detecting aparasitic load based on the proportionality constant being greater thana threshold.
 12. The method of claim 1, further comprising in accordancewith detecting a receiver based on the proportionality constantsatisfying a predetermined parameter, transmitting wireless power to thereceiver.
 13. The method of claim 1, further comprising, in accordancewith detecting a receiver based on the proportionality constantsatisfying a predetermined parameter, identifying whether the receiveris a receiver that is compatible with the inductively coupled powertransfer system.
 14. The method of claim 1, further comprising, inaccordance with detecting a receiver based on the proportionalityconstant satisfying a predetermined parameter, switching a transmitterof the system to a standby mode.
 15. The method of claim 1, furthercomprising, in accordance with detecting a receiver based on theproportionality constant satisfying a predetermined parameter, switchinga transmitter of the system off.
 16. An inductive power transmittercomprising: a coil for generating an alternating magnetic field; aconverter for supplying an alternating current to the coil; a sensor formeasuring an indication of the current supplied by the converter; and acontroller configured to: operate the converter in a power delivery modeto deliver power to an inductive power receiver via the coil; switch theconverter to a power detection mode for detecting inductive powerreceivers or parasitic loads while transferring power, wherein, in thepower detection mode, the controller is configured to: operate theconverter so as to vary at least one of a voltage or frequency ofalternating current supplied to the coil to a plurality of test voltagesor test frequencies; using the sensor, detecting a change in currentsupplied to the coil associated with the varying of at least one of thevoltage or frequency of alternating current supplied to the coil to theplurality of test voltages or test frequencies; perform a regressionanalysis of the change in current associated with the plurality of testvoltages or test frequencies to determine a proportionality constantrelating the change in current to the variation of at least one of avoltage or frequency of alternating current supplied to the coil; anddetect a receiver or a parasitic load based on the proportionalityconstant satisfying one or more predetermined parameters.
 17. Theinductive power transmitter of claim 16 wherein the controller isconfigured to operate the converter so as to vary at least one of avoltage or frequency supplied to the coil by adjusting the frequency toone or more test frequencies.
 18. The inductive power transmitter ofclaim 17 wherein each of the one or more test frequencies is less than4% different than an operating frequency in the power delivery mode. 19.The inductive power transmitter of claim 17 wherein: the controller isconfigured to detect a change in current supplied to the coil by:measuring average steady state current supplied at the power deliveryfrequency; storing the measured average steady state current andassociated power delivery frequency; measuring average steady statecurrent supplied at the one or more test frequencies; and storing themeasured average steady state current and associated test frequencies;and wherein the controller is configured to perform a regressionanalysis of the change in current to determine a relationship betweensteady state current and frequency.
 20. The inductive power transmitterof claim 17, wherein the controller is further configured to detect areceiver based on the proportionality constant being greater than athreshold.
 21. The inductive power transmitter of claim 17, wherein thecontroller is further configured to detect a parasitic load based on theproportionality constant being less than a threshold.
 22. The inductivepower transmitter of claim 16 wherein operating the converter so as tovary at least one of a voltage or frequency supplied to the coilcomprises adjusting the voltage to one or more test voltages.
 23. Theinductive power transmitter of claim 22 wherein each of the one or moretest voltages is less than 4% different than an operating voltage in thepower delivery mode.
 24. The inductive power transmitter of claim 22wherein: the controller is configured to detect a change in currentsupplied to the coil by: measuring average steady state current suppliedat the power delivery voltage; storing the measured average steady statecurrent and associated power delivery frequency; measuring averagesteady state current supplied at the one or more test frequencies; andstoring the measured average steady state current and associated testfrequencies; and wherein the controller is configured to perform aregression analysis of the change in current to determine a relationshipbetween steady state current and voltage.
 25. The inductive powertransmitter of claim 22, wherein the controller is further configured todetect a receiver based on the proportionality constant being greaterthan a threshold.
 26. The inductive power transmitter of claim 22,wherein the controller is further configured to detect a parasitic loadbased on the proportionality constant being greater than a threshold.27. The inductive power transmitter of claim 16, wherein the controlleris further configured to, in accordance with detecting the receiverbased on the proportionality constant satisfying a predeterminedparameter transmit wireless power to the receiver.
 28. The inductivepower transmitter of claim 16, wherein the controller is furtherconfigured to in accordance with detecting the receiver based on theproportionality constant satisfying a predetermined parameter identifywhether the receiver is a receiver that is compatible with theinductively coupled power transfer system.
 29. The inductive powertransmitter of claim 16, wherein the controller is further configuredto, in accordance with detecting the receiver based on theproportionality constant satisfying a predetermined parameter, switchthe transmitter to a standby mode.
 30. The inductive power transmitterof claim 16, wherein the controller is further configured to, inaccordance with detecting the receiver based on the proportionalityconstant satisfying a predetermined parameter, switch the transmitteroff.